This isn't a comparison between iMac and Quad G5, but the Quad G5 does OK on Universal apps--the Mac Pro is obviously faster, and significantly so, but that suggests to me a Quad G5 is roughly as fast as a new iMac, which means it's no slouch. I'd be curious to see a comparison between a Quad G5 running CS2 and a Mac Pro running CS3. (Yes, I understand the comparison would be inexact, but I think it would be quite informative!).
Like someone else mentioned, "faster" depends on what you are doing. I can't tell a difference between my 2,8 ghz iMac and my 1,67 ghz pb G4 in iTunes, Safari, Mail, iCal,... even opening vector graphics in Illustrator CS3 or not too complex documents in InDesign CS3 is about the same speed.
The difference becomes noticeable when Photoshop documents go beyond 150 - 200 mb, which they often do.
Or when rendering a complex image in Cinema 4D, the iMac will do it in 20 seconds where the pb needs 3 minutes.
And of course when converting homemade movies, where a 6 hours job on the pb lasts 30 minutes on the iMac.